Ferenc Zsoldos

Chloroplastic glutamine synthetase is activated by direct binding of aluminium

Acidification of soils may release water soluble, toxic aluminium species from clay minerals. Al interferes with a wide range of physical and cellular processes. Glutamine synthetase (GS, EC 6.3.1.2) is the key enzyme of primary N assimilation, as well as ammonia reassimilation and detoxification. Plant GS requires two magnesium ions per subunit for activity, which makes GS a potential target of metal stress. The objective of this investigation was to prove that Al from an organic metal complex is able to activate GS, and Al becomes bound to the polypeptide structure of the GS molecule. Aluminium(III)-nitrilotriacetic acid complex (Al(III)NTA) activated the GS prepared from wheat (Triticum aestivum L.) leaves, as Al(3+) did in vivo, but could not functionally substitute magnesium ions, which were also necessary for the activity in the in vitro GS assay. GS2 was isolated by non-denaturing polyacrylamide gel electrophoresis, and the Al and Mg content of the enzyme was determined by inductively coupled plasma atomic emission spectroscopy. The GS octamer remained intact and contained Mg(2+) bound to its specific sites after the electrophoretic separation. Al was detected in the Al(III)NTA-treated sample bound to the structure of the enzyme protein, potentially occupying one of the specific metal-binding sites of the subunits. Our results indicate that the activatory effect of the Al(III)NTA complex is because of specific binding of aluminium to the polypeptide chain of GS2, however presence of magnesium at least on one of the metal-binding sites is essential to the active state of the enzyme.

Growth of and potassium transport in winter wheat and durum wheat as affected by various aluminum exposure times.

Abstract Effects of various aluminum (AlCl3) concentration and exposure times (6, 12, and 24 h and 3 d) on growth and potassium (K) transport were studied in two wheat species (Triticum aestivum L. cv. Jubilejnaja 50 and Triticum durum Desf. cv. GK Betadur) grown in low salt conditions hydroponically. In longer (3 d) Al exposure times at pH 4.1, the inhibition of root growth appeared at 10 μM Al3+ treatment in GK Betadur, and at 50 μM Al3+ treatment in Jubilejnaja 50. Shoot growth was not influenced by Al3+ treatment, except at 100 μM in 7 d experiments. In 6, 12, and 24 h Al3+ exposure times, at low pH, the K+(86Rb) influx in roots increased as the Al3+ concentration increased in the outer medium in both species. It also appeared in K+(86Rb) transport toward the shoots, except by higher Al3+ treatments of GK Betadur seedlings. At the same time, in longer‐term (3 d) Al3+ treatments, a striking inhibition were observed in K+(86Rb) influx and K+ concentration of roots and shoots. The K+concentration of roots and shoots measured at the end of 24 h Al3+ exposure times was significantly not affected by Al3+ treatment. Durum wheat proved to be more sensitive to the Al toxicity than common winter wheat.

Effects of pH and nitrite on potassium uptake and growth of rice seedlings

Summary The effects of increasing concentrations of NaNO2 on the K+ uptake of rice seedlings (Oryza sativa L. cv. Oryzella) were studied at different pH values. Increasing concentrations of NaN02 caused decreases in the K+ uptake of the roots. When NaNO3 was present in the uptake solution, marked changes were not observed in the K+ uptake. The inhibitory effect of NO2- was strongly influenced by the H+ concentration of the outer medium. A decrease of the pH of the external solution led to an increased inhibitory effect of NO2- on both the ion uptake and the growth of the seedlings. However, the reverse was true in the case of Na+ uptake. The inhibitory effect of NO2- was different for roots and shoots. Marked changes in the K+ efflux properties were not observed at pH 6. A decrease of the pH of the efflux medium, however, led to an increased loss of K+ from the roots. The results strongly suggest a distinct role of NO2- in membrane damage at lower pH.

GROWTH AND POTASSIUM TRANSPORT IN COMMON AND DURUM WHEAT AS AFFECTED BY ALUMINUM AND NITRITE STRESS

Aluminum (Al) toxicity has been identified as one of the most important factors limiting plant growth in acid soil. Besides Al, nitrite (NO2 −) may also be a significant stress factor in an acid environment. The objective of this study was to examine the effects of Al and NO2 − stress on the growth and potassium (K+) uptake of roots and their transport toward the shoots of an Al-resistant common wheat (Triticum aestivum L. cv. Jubilejnaja 50) and an Al-sensitive durum wheat (T. durum Desf. cv. GK Betadur) grown in 0.5 mM CaSO4 solution at pH 4.1 or 6.5. Root elongation of durum wheat was inhibited with 30% at 10 μM AlCl3 treatment, while this low Al-concentration did not show a significant effect on root growth of common wheat. In all cases shoot growth was not influenced under low-salt conditions by 10 μ M AlCl3, but exposure to 100 μM KNO2 (alone or in combination with Al) had a definite stimulatory effect on growth. Aluminum was found to stimulate the K+(86Rb) influx in short-term (6 h) experiments, but to inhibit it in long-term (3 days) experiments. This treatment was thought to damage the plasma membrane. When 10 μM 2,4-dinitrophenol was present in the uptake solution the Al-stimulated K+ uptake stopped even in short-term experiments. In the case of nitrite and nitrite + Al treatment combinations, however, a striking inhibition was observed in the K+(86Rb) influx and the K+ concentration of the roots and shoots of both species.